The Digital Photography Book by Scott Kelby

http://www.scottkelbybooks.com

Michigan Tech – Computer Science

http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/filter/filter-ND.html

http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/filter/polarizer.html

great-landscape-photography.com

http://www.great-landscape-photography.com

ePHOTOzine

http://www.ephotozine.com/article/Using-a-Neutral-Density-filter

Depth of Field – Wikipedia, the free encyclopedia

http:///www.en.wikipedia.org/wiki/Depth_of_field

The Luminous Landscape

http://www.luminous-landscape.com/tutorials/polarizers.shtml

Digital Photography School

http://digital-photography-school.com/blog/how-to-use-and-buy-polarizing-filters/

Edbergphoto.com

http://www.edbergphoto.com/pages/Tip-polarizers.html